Method intended for detection and automatic classification, according to various selection criteria, of seismic events in an underground formation

ABSTRACT

Method intended for automatic classification, according to various selection criteria, of seismic or microseismic events picked up by seismic receivers coupled with an underground formation under development, according to whether the events are purely of microseismic nature or depend, on any account, on artifacts related to the outer environment or to formation development or monitoring activities, and for selection of the significant parts of traces for the purpose of analysis.  
     It comprises detecting, on at least one recorded trace, during a detection window (Pld) and according to at least one criterion, seismic signals that exceed a given threshold (Sd) in relation to a determined selection function (E, A), selecting, on at least this trace, at least one global recording interval (Ple) overlapping into either side of detection interval (Pld), and storing the signals of this recording interval (Ple) with a determined classification label, according to whether the signals detected therein correspond purely to a microseismic event or are related, to a certain extent, to artifacts.  
     Application: monitoring of production or storage reservoirs under development.

FIELD OF THE INVENTION

[0001] The present invention relates to a method intended for automaticclassification, according to various selection criteria, of records ofseismic or microseismic events picked up by seismic receivers coupledwith an underground formation under development.

[0002] The method according to the invention is notably applied formonitoring of reservoir zones generally used either to extract or toinject fluids therein.

BACKGROUND OF THE INVENTION

[0003] Locating points of an underground zone, whether a reservoir zoneor a cavity, in which occur microseismic events related to an activitywhich has the effect of modifying the stress field, is of greatsignificance for the development of the zone, whether production offluids extracted from a reservoir through one or more wells or injectionof fluids into the zone.

[0004] It is for example possible to monitor the evolution of ahydrocarbon reservoir under development or of geothermal sites. In thecase of enhanced recovery notably, it is desired to sweep the oil out ofthe rock by injecting fluids at pressures and temperatures that may bevery different from those of the medium. The resulting stress variationsmay lead to a fracturation of the medium, which modifies the circulationof the fluids inside the reservoir and which it is important to locateproperly.

[0005] It is also well-known to use underground reservoirs for fluidstorage. The latter may be, for example, liquid or gas phase storagereservoirs wherein a certain microseismic activity induced bysignificant variations in the flow rate of the liquid or the gas takenfrom or injected into these reservoirs can be observed.

[0006] The reservoirs used may also be reservoir zones or cavities usedfor waste storage, which have to be monitored so as to respect theenvironment and to comply with the increasingly restricting pollutioncontrol regulations. An underground reservoir can be used to injectdrilling fluids containing more or less large amounts of solidparticles, dumping these fluids after use on drilling sites beingforbidden by the regulations.

[0007] The temperature of the fluids injected is generally verydifferent from the temperature of the medium at the depth where they areinjected, which, in case of massive injection, has the effect ofproducing thermal stresses which generate fractures and therefore leadto a certain seismic activity. The injection pressure of these muds canalso create stresses and lead to more or less great changes in themedium.

[0008] The seismic activity induced by the effects of the pressure or ofthe temperature can reveal, for example, the formation of fractures orgive rise to stresses in already existing fractures. They contribute tomodifying the flow paths of the fluids in the medium, or they createescape paths for fluids out of the reservoir, containment breaks withpossible pollution of the surrounding zones, notably of an aquifer usedfor drinkable water supply, which it is important to detect.

[0009] It is also of very great significance to monitor reservoirs usedfor storage of nuclear waste so as to prevent injection operations orlocal temperature rises due to storage from causing fractures in thelayers providing containment.

[0010] With microseismic monitoring, also known as passive seismic, thedeveloper's aim is, in the end, to interpret very quickly the data inconnection with the conventional working data (pressure, flow rate,temperature, etc.) so as to be able to take account of the mechanicalresponse of the site in the development protocol in order to preservethe productivity of the well(s) or of the site. The microseismicactivity observed can be combined with a mechanical degradation of themedium which can generate solids encroachment, with the opening offractures likely to communicate the reservoir with an aquifer, or withother phenomena of thermo-poro-mechanical origin whose consequences maylead to a performance degradation of the wells or even to well damage.

[0011] Patents FR-2,593,292 (U.S. Pat. No. 4,775,009), FR-2,681,573(U.S. Pat. No. 5,303,773) or EP-546,892 (U.S. Pat. No. 5,370,445)notably describe various techniques for monitoring the evolution overtime of underground reservoirs, which comprise using seismic or otherpickups permanently installed in one or more wells (embedded in thecement coupling the casing with the formation or externally combinedwith a production tubing and pressed against the inner face of thecasing) without disturbing the various operations carried out(production, injection, various servicing operations performed by meansof these wells). Permanent installation of pickups in wells allowsseismic monitoring of a reservoir in order to detect different phenomenarelated to the development thereof.

[0012] Patents FR-2,703,457, FR-2,703,470 or EP-748,457 (U.S. Pat. No.5,724,311) describe methods intended for long-term repetitive activemonitoring of a reservoir through application of elastic waves to aformation and acquisition of the response signals sent back by theformation, by means of the permanent installation of emission andreception means in wells or close to the ground surface. Differentialprocessing operations are carried out on acquisitions performed underidentical conditions.

[0013] Patents FR-2,688,896, FR-2,689,647 (U.S. Pat. No. 5,481,502)notably describe electronic acquisition and transmission systemsspecially designed to collect signals from permanent pickups installedin wells outside casings or production tubings, and to transmit them toa surface recording and control equipment during long-term repetitivemonitoring or seismic surveys.

[0014] Patent application EP-A-1,074,858 and patent FR-2,780,900 (U.S.Pat. No. 6,113,388) also describe a method intended for automaticanalysis of signals acquired by one or more elastic or acoustic wavepickups so as to automatically locate thereon, with high accuracy, atleast one significant time such as the time of first arrival on eachpickup and/or the end of these signals.

[0015] Patent FR-2,772,137 (U.S. Pat. No. 6,049,508) describes a methodintended for automatic discrimination, from among all the eventsrecorded, of those of type E which are of interest for characterizationof the site, while taking into account the events induced at completionlevel, which are referred to as type C events. These events aregenerated by injection stops and resumptions, opening or closing of oneof the completion elements (valve, packer, etc.) at any depth in thewell or even at the surface (at the wellhead), including at surfaceinstallation level (pipes, various devices). Some of these actions, suchas communication of the underground zone (reservoir) with the surfacenetwork, may lead, as a result of pressure variations notably, to type Eevents, often deferred, which it is desired to record and interpret.Type C events, which can be in very great number within a relativelyshort time interval (more than 3400 events in one week, by way ofpractical example), are detrimental to real-time monitoring of thegeomechanical phenomena induced through the acquisition of type E eventswhich are often in relatively small number for the same period (some tenevents for example).

[0016] Besides the seismic receivers coupled with the formation, one ormore reference pickups are used, which provide direct acoustic couplingwith elements of the technical zone development equipment for detectionof the elastic waves directly linked with the development. By means of acomparative analysis of the signals coming from these receivers and fromeach reference pickup, the records are sorted into different familiesaccording to whether the events in the underground zone are independentof the events detected by each reference pickup or depend on themdirectly or indirectly.

[0017] Systematic listening to microseismic events likely to occur in anunderground formation generates, as mentioned above, a large amount ofseismic records that have to be carefully classified so as to simplifylater discrimination and analysis work.

SUMMARY OF THE INVENTION

[0018] The method according to the invention allows automaticclassification, according to various criteria, of events detected onrecorded seismic traces corresponding to signals picked up by seismicreceivers coupled with an underground formation, according to whetherthe events are purely of microseismic nature or depend, on any account,on artifacts related to the outer environment or to formationdevelopment or monitoring activities, and selection of the significantparts of traces for the purpose of analysis.

[0019] It is essentially characterized in that it comprises detecting,on at least one record trace during a detection window (Pld) andaccording to at least one criterion, seismic signals which exceed acertain threshold (Sd) in relation to a determined selection function(E, A), selecting, at least on said trace, a global recording interval(Ple) overlapping into either side of detection interval (Pld), andstoring the signals of this recording interval (Ple) with a determinedclassification label, according to whether the signals detected thereincorrespond purely to a microseismic event or are related, to a certainextent, to artifacts.

[0020] According to an implementation mode, the method comprisesapplying a selection criterion including the detection of events on asingle record trace (Tn) and storing corresponding global recordingintervals on several traces.

[0021] According to another implementation mode, the method comprisesapplying a selection criterion including the detection of events on acertain number p (p≧2) of different record traces (Tn, Tp) if thecorresponding detection intervals (Pld) are included in a determinedtime interval (Plg), by applying the same selection function, andstoring a global recording interval of sufficient duration to cover allthe events, at least on said p traces.

[0022] According to another implementation mode, the method comprisesapplying a selection criterion including the detection of events on acertain number p (p≧2) of different record traces (Tn, Tp) if thecorresponding detection intervals (Pld) are included in a determinedtime interval (Plg), by applying the same selection function, andstoring a global recording interval of sufficient duration to cover theevents, on a number N of traces greater than number p.

[0023] According to another implementation mode, when one of said ptraces is produced by acquisition of signals due to artifacts, themethod comprises applying a cancellation criterion comprising theclassification of the other traces where events are detected in thecategory of the cancelled traces, and storing them.

[0024] According to another implementation mode, the method comprisesinhibiting the traces for which the detection time is longer than adetermined time interval and revalidating them later if the value of theselection function is below a second threshold in a determinedreactivation interval.

[0025] The seismic traces classified with the method are obtained bymeans of seismic or acoustic pickups coupled with the formationssurrounding a well through the underground formation. They cancorrespond to signals reflected by the formation in response to theemission, in the ground, of signals emitted by a seismic source and, inthis case, inhibition criteria are preferably used on several traces soas to select only seismic events that are not caused by said source.

[0026] The data processing device according to the invention allows toautomatically classify, according to different criteria, events detectedon recorded seismic traces corresponding to signals picked up by seismicreceivers coupled with an underground formation, according to whetherthe events are purely of microseismic nature or depend, on any account,on artifacts related to the outer environment or to formationdevelopment or monitoring activities, in order to select significantparts of traces for the purpose of analysis. It essentially comprisesmeans for detecting, on at least one record trace during a detectionwindow and according to at least one criterion, seismic signals whichexceed a certain threshold in relation to a determined selectionfunction, means for selecting at least on said trace a global recordinginterval overlapping into either side of the detection interval, andmeans for storing the signals of this recording interval with adetermined classification label, according to whether the signalsdetected therein correspond purely to a microseismic event or arerelated, to a certain extent, to artifacts.

[0027] The system intended for seismic monitoring of an undergroundformation according to the invention essentially comprises a pluralityof seismic receivers coupled with the formation, a device intended torecord the signals picked up by the various seismic receivers and theprocessing device defined above.

[0028] The seismic receivers are, for example, hydrophones and/orgeophones and/or accelerometers distributed in at least one well throughthe formation that can be coupled with the formation by means of aliquid or cement, mechanical coupling means or by magnetization.

[0029] The device can comprise at least one transmission cableconnecting the seismic receivers to the recording device.

[0030] The data reception and/or transmission means use for example atechnology based on optical fibers.

[0031] The device can include means for inhibiting several record tracesso as to select only seismic events that are not caused by said sourceor to prevent saturation of the recording device.

[0032] By means of these detection and classification protocols, themethod and the device allow to isolate, from among the oftenconsiderable amount of record files obtained through systematiclistening to the microseismicity of a formation, a much more limitednumber of files containing directly interpretable, dated and classifiedsignificant records.

[0033] Unnecessary saturation of the data storage capacities is thusprevented and the effective autonomy of the seismic event processingdevice, whose capacity to work under stand-alone or independentconditions is a great asset, is increased.

[0034] The possibility of working with several modes simultaneouslyprovides the processing device with certain advantages for themanagement of large sites where the zones monitored can undergomechanical readjustments that are not necessarily perceptible from onezone to the next.

[0035] The possibility of temporarily and automatically inhibitingcontinuous detections on certain seismic receivers, used in combinationwith a mode of operation using several detection/inhibition modes,notably allows not to lose any useful information that would be acquiredthrough certain reception channels listened to only intermittently orthrough these channels when they meet the reactivation criteria.

BRIEF DESCRIPTION OF THE FIGURES

[0036] Other features and advantages of the method according to theinvention will be clear from reading the description hereafter of anon-limitative example, with reference to the accompanying drawingswherein:

[0037]FIG. 1 diagrammatically shows a well equipped with receivers forseismic listening in a formation,

[0038]FIG. 2 diagrammatically shows a method of fastening seismicreceivers to a tubing,

[0039]FIG. 3 illustrates the criterion applied to the detection of asingle trace,

[0040]FIG. 4 illustrates a variant of this criterion allowing toseparate the successive events,

[0041]FIG. 5 illustrates the criterion applied to the detection appliedto several traces, and

[0042]FIG. 6 illustrates the trace inhibition and reactivation criterionallowing to reject too noisy traces.

DETAILED DESCRIPTION

[0043] The seismic or microseismic monitoring system diagrammaticallyshown in FIG. 1 is installed in one or more wells 1 provided each with acasing pipe 2 which, once in place, is coupled with the surroundingformations by injecting cement between the pipe and the well. Aproduction tubing 3 is lowered into the cased well. This tubing allowsto carry out operations in an underground zone P (in order to extractfluids therefrom or to inject fluids therein). Pumping means and valves(not shown) allow controlled activation of the zone considered. Theseismic monitoring system comprises for example, in each well, a seriesof n seismic receivers R1 to Rn fastened to the exterior of the tubingand pressed against the inner wall of casing 2 by means of uncouplingmeans such as flexible elements 4 (FIG. 2). It can also includereceivers.

[0044] Each receiver Rk is for example a triphone comprising threegeophones in a single box, whose axes are oriented in the directions ofa trirectangular trihedron. Local electronic boxes 5 are arranged in thewell. Each one is connected to one or more triphones Rk for acquisitionof the signals picked up by the associated receivers. The various localelectronic boxes are connected by one or more transmission cables C to asurface collection device 6 such as a microcomputer. In cases wherereceivers are permanently installed in several wells, at a distance fromone another, the various collection devices 6 are for example connectedto a distant central laboratory (not shown) by means of a communicationnetwork such as an Ethernet network for example, as described forexample in the aforementioned patents FR-2,772,137, FR-2,689,647 (U.S.Pat. No. 5,481,502). A multiwell multilevel telemetry system cancomprise for example up to nine or eighteen levels of three traces pertelemetric link installed in a well, and up to ten equipped wells.

[0045] In cases where a single telemetric link is used per well, alistening session can be launched from each surface assembly or from thecentral laboratory by means of an activation signal. Once a day or onthe operator's initiative, listening is interrupted and a series oftests is carried out according to the protocol described hereafter inorder to locate, on the records, the notable seismic events that haveoccurred during the listening session, to classify them and to recordthem in the journal, after which listening is restarted.

[0046] Listening Configuration

[0047] This action allows to select, from the site equipmentconfiguration, the listening configuration, i.e. all of the recordtraces (referred to as <<listening>> traces) that will be part of thelistening session.

[0048] This stage being completed, the operator can define theacquisition configuration.

[0049] Definition of the Detection Criteria

[0050] This operation allows the operator to define the detectioncriteria relative to the microseismic events and all of the listeningtraces on which they will be applied. A certain number of tracestherefore has to be defined among the listening traces:

[0051] the <<detecting>> traces on which the criteria are applied,

[0052] the traces to be recorded, which will be recorded on eachcriterion, and

[0053] the cancellation traces that will allow to confirm or invalidatethe fact that an event detected on one or more detecting traces is amicroseismic event.

[0054] If a microseismic event is detected, all the digitized signals onall the traces to be recorded at times that delimit the event are datedand stored on the disk of the acquisition microcomputer. The journal isalso updated.

[0055] By definition, a criterion is an analysis function to be appliedon one or more traces within a given period of time.

[0056] It is for example an <<energy >> function E(Tn, Pld, Sd). Thisfunction calculates the rms value of the signal of trace Tn in ainterval Pd and compares the result with a threshold Sd. If thethreshold value is exceeded, the function returns the Boolean TRUE; inthe opposite case, it returns FALSE. The rms value is calculated bytaking away from each sample the continuous component calculated ininterval Pld.

[0057] It can also be an <<amplitude>> function A(Tn, Pld, Sd, X). Thisfunction calculates the absolute value of the signal of trace Tn in ainterval Pld and compares the number of times threshold Sd is exceededin relation to number X used as a parameter. If the threshold isexceeded, the function returns the Boolean TRUE; in the opposite case,it returns FALSE.

[0058] We are going to list hereunder a certain number of eventdetection criteria, then of event cancellation criteria and finally aprocedure allowing a noisy detecting trace to be inhibited.

[0059] Recording Limits

[0060] In order to prevent saturation of the storage disk, it may bedecided for example to record only 50% of the events over a period ofone hour and 10% over one day. This limitation can be removed forcertain operations such as listening during fracturation. Duringpermanent listening, all the events are automatically backed up atregular intervals (every 24 hours for example on an optical disk). Thisbackup procedure can also be performed at the operator's request.

[0061] Various Detection Criteria

[0062] Principle of the Criteria Applied to a Single Trace Tn (FIGS. 3,4)

[0063] This criterion allows to detect that an event is a microseismicevent. For a trace Tn, the criterion can be formulated as follows: adetected event becomes microseismic if the result of the functionapplied to interval Pld (detection interval) exceeds threshold Sd. Thedetected event causes recording of all the traces to be recordedassociated with this criterion. This recording procedure starts on thesamples acquired from the start of interval Pld minus an interval Ple1(front recording interval) and involves an interval Ple2 (totalrecording interval). For detection of the threshold, interval Pldprogresses each time by a half-interval.

[0064] If a new detection occurs before the end of an interval Plg(global event interval), one considers that it is the same microseismicevent. On the other hand, any new detection beyond Plg is considered tobe another microseismic event and it is recorded.

[0065] All the recorded traces have a size Ple ranging from several tenmilliseconds up to one or more seconds for example. In theabove-mentioned case, the records overlap.

[0066] Principle of the Multitrace Detection Criterion (FIG. 5)

[0067] This type of criterion allows to confirm more finely that anevent is actually microseismic. The traces on which these criteria areapplied are obtained in a single well or in different wells. For twotraces Tn and Tp, the criterion can be formulated as follows:

[0068] The detected event becomes microseismic if, in a global intervalPlg, the rms value of Tn in an interval Pld exceeds threshold Sdn andthe rms value of Tp in an interval Pld exceeds threshold Sdp. Thedetected event causes recording of all the traces to be recordedassociated with this criterion. This recording procedure starts on thesamples acquired at the beginning of interval Pldn minus an intervalPle1 and involves an interval Ple2.

[0069] If the criterion is again met before the end of interval Plg, oneconsiders that it is the same microseismic event. On the other hand, anynew detection beyond Plg is considered to be another microseismic eventNes and it is recorded.

[0070] This criterion can concern more than two traces, and validationcan occur either:

[0071] if N detecting traces meet the criterion in a global intervalPlg, or

[0072] if N traces among the detecting traces at least meet thecriterion in a global interval Plg.

[0073] Criterion Types and Parameterization

[0074] Parameterization by Traces

[0075] For each detecting trace, the following parameters are defined:

[0076] the function to be applied, E or A,

[0077] the detection threshold Sd,

[0078] the inhibition threshold Si,

[0079] the trace reactivation threshold Sr,

[0080] the exceeding number X (if the function selected is<<amplitude>>).

[0081] Criterion Relative to N Specific Detecting Traces (Criterion Ca)

[0082] This type of criterion is met if the result of all the functionsperformed on all the detecting traces specified returns the Boolean TRUEin a given time interval, which amounts to an AND logic operationbetween all the function results.

[0083] The following parameters are required to parametrize thiscriterion (Ca):

[0084] list of the detecting traces related to this criterion,

[0085] detection limit interval (Plg),

[0086] detection calculation interval (Pld),

[0087] list of the record traces related to this criterion, frontrecording interval (Ple1), and

[0088] recording time (Ple2).

[0089] During a listening session, the operator can define several typeCa criteria with different parameters (Ca1, Ca2, etc.).

[0090] Criterion Relative to N Detecting Traces Among P (Criterion Cb)

[0091] This type of criterion is met if the result of at least Nfunctions carried out on all the detecting traces specified returns theBoolean TRUE in a given time interval.

[0092] The following parameters are required to parametrize thiscriterion (Cb):

[0093] list of the detecting traces related to this criterion,

[0094] number of traces that must meet the criterion,

[0095] list of the record traces related to this criterion,

[0096] detection limit interval (Plg),

[0097] detection calculation interval (Pld),

[0098] front recording interval (Ple1), and

[0099] recording time (Ple2).

[0100] During a listening session, the operator can define several typeCb criteria with different parameters (Cb1, Cb2, etc.).

[0101] Criterion Relative to a Single Detecting Trace (Criterion Cc)

[0102] This criterion is met if the result of the function carried outon one of the specified traces returns the Boolean TRUE, which amountsto an OR logic operation between all the results of the functionsassociated with the traces.

[0103] The following parameters are required to parametrize thiscriterion (Cc):

[0104] number of the detecting trace(s),

[0105] detection limit interval (Plg),

[0106] detection calculation interval (Pld),

[0107] list of the record traces related to this criterion,

[0108] front recording interval (Ple1), and

[0109] recording time (Ple2).

[0110] During a listening session, the operator can define several typeCc criteria with different parameters (Cc1, Cc2, etc.). He can also givea list of detecting traces using the same parameters.

[0111] Event Cancellation Criterion (Cs)

[0112] When a detection criterion is being defined, a certain number oftraces is selected to be cancellation traces. These traces are generallythe surface traces or the tubing traces. In case of cancellation of thedetection criterion, only the cancellation traces will be recorded. Thiscancellation criterion is used to detect a surface noise caused by workin progress.

[0113] The following parameters are required to parametrize thiscriterion (Cs):

[0114] number of the surface trace (Ts) or of the tubing trace (Tb),

[0115] detection calculation interval (Plad),

[0116] front cancellation interval (Pla1) (positive or negative value),

[0117] total cancellation interval (Pla2),

[0118] front recording interval (Plea1), and

[0119] recording time (Plea2).

[0120] If the cancellation criterion is triggered, detection of theassociated criterion is cancelled during interval Plea2.

[0121] Trace Inhibition and Reactivation Criterion (Ci) (FIG. 6)

[0122] This criterion allows to inactivate too noisy a detecting trace.

[0123] In any case, if an event detection on a detecting trace extendsover an interval greater than Pli (inhibition interval), this trace isno longer taken into account in the detection criteria. It will bevalidated again if its rms value remains below the reactivationthreshold (Sr) during an interval Plr (reactivation interval). Thisprinciple allows to eliminate the non-seismic events or even continuousseismic events resulting from the temporary use of a seismic source foranother purpose, such as an isolated seismic acquisition or acquisitionsof the same type carried out regularly.

[0124] An inhibited trace is removed from the list of the detectingtraces. If it was part of a type Ca (AND) criterion, its triggering willstill be considered as TRUE.

[0125] Example for a Detecting Trace

[0126] If the natural noise level is 2μ Veff, one can take:

[0127] a 100-ms detection interval (Pld),

[0128] a 4μ Veff detection threshold (Si),

[0129] a 2-second inhibition interval (Pli),

[0130] a 2-second reactivation interval (Plr),

[0131] a 3μ Veff reactivation threshold to reactivate the trace in thecriterion (Sr).

[0132] The following parameters are required to parametrize thiscriterion (Ci):

[0133] number of traces subjected to this principle,

[0134] inhibition interval (Pli),

[0135] reactivation interval (Plr).

[0136] The inhibition and reactivation thresholds are the thresholds Siand Sr of each detecting trace.

[0137] The method has been described in applications to seismic ormicroseismic data processing. It is clear that it can be applied moregenerally for processing any signal of undulatory or acoustic nature.

1) A method intended for automatic classification, according to variouscriteria, of events detected on recorded traces corresponding to seismicor acoustic signals picked up by receivers (R1-Rn) coupled with anunderground formation, according to whether the events are purely ofseismic or microseismic nature or depend, on any account, on artifactsrelated to the outer environment or to formation development ormonitoring activities, and for selection of the significant parts oftraces for the purpose of analysis, characterized in that it comprisesdetecting, on at least one record trace during a detection window (Pld)and according to at least one criterion, seismic signals that exceed acertain threshold (Sd) in relation to a determined selection function(E, A), selecting, at least on said trace, a global recording interval(Ple) overlapping into either side of detection interval (Pld), andstoring the signals of this recording interval (Ple) with a determinedclassification label, according to whether the signals detected thereincorrespond purely to a seismic or microseismic event, or are related, toa certain extent, to artifacts. 2) A method as claimed in claim 1,characterized in that it comprises applying a selection criterioncomprising detecting events on a single record trace (Tn) and storingcorresponding global recording intervals on several traces. 3) A methodas claimed in claim 1, characterized in that it comprises applying aselection criterion comprising detecting events on a certain number p(p≧2) of different record traces (Tn, Tp) if the corresponding detectionintervals (Pld) are included in a determined time interval (Plg), byapplying the same selection function, and storing a global recordinginterval of sufficient duration to cover the events, at least on said ptraces. 4) A method as claimed in claim 1, characterized in that itcomprises applying a selection criterion comprising detecting eventsover a certain number p (p≧2) of different record traces (Tn, Tp) if thecorresponding detection intervals (Pld) are included in a determinedtime interval (Plg), by applying the same selection function, andstoring a global recording interval of sufficient duration to cover theevents, on a number N of traces greater than number p. 5) A method asclaimed in claim 3, characterized in that one of said p traces beingproduced by aqcuisition of signals due to artifacts, it comprisesapplying a cancellation criterion comprising classifying the othertraces where events are detected in the category of the cancelled tracesand storing them. 6) A method as claimed in any one of the previousclaims, characterized in that it comprises inhibiting traces for whichthe detection time is greater than a determined time interval (Pli) andlater revalidating them if the value of the selection function (E, A) isbelow a second threshold in a determined reactivation interval (Plr). 7)A method as claimed in any one of the previous claims, characterized inthat seismic traces obtained by means of seismic or acoustic pickupscoupled with the formations surrounding a well through the undergroundformation are used. 8) A method as claimed in any one of the previousclaims, characterized in that several selection criteria selected fromamong all the criteria of a criteria library are used. 9) A method asclaimed in any one of the previous claims, characterized in that thetraces recorded are the signals sent back by the formation in responseto the emission in the ground of signals emitted by a seismic source,and in that inhibition criteria are used on several traces so as toselect only seismic events that are not caused by said source. 10) Adata processing device intended for automatic classification, accordingto various criteria, of events detected on recorded seismic tracescorresponding to signals picked up by seismic receivers (Rk) coupledwith an underground formation, according to whether the events arepurely of microseismic nature or depend, on any account, on artifactsrelated to the outer environment or to formation development ormonitoring activities, and for selection of significant parts of tracesfor the purpose of analysis, characterized in that it comprises meansfor detecting, on at least one record trace in a detection interval(Pld) and according to at least one criterion, seismic signals thatexceed a certain threshold (Sd) in relation to a determined selectionfunction (E, A), means for selecting, at least on said trace, a globalrecording interval (Ple) overlapping into either side of detectioninterval (Pld), and means for storing signals of this recording interval(Ple) with a determined classification label, according to whether thesignals detected therein correspond purely to a microseismic event orare related, to a certain extent, to artifacts. 11) A device intendedfor seismic monitoring of an underground formation, comprising aplurality of seismic receivers (Rk) coupled with the formation, a device(6) for recording the signals picked up by the various seismic receiversand a device (7) for processing the signals recorded so as toautomatically classify, according to various criteria, events detectedon the recorded seismic traces corresponding to signals picked up by theseismic receivers coupled with an underground formation, according towhether the events are purely of microseismic nature or depend, on anyaccount, on artifacts related to the outer environment or to formationdevelopment or monitoring activities, and selection of the significantparts of traces for the purpose of analysis, characterized in that itcomprises means for detecting, on at least one record trace in adetection window (Pld) and according to at least one criterion, seismicsignals that exceed a certain threshold (Sd) in relation to a determinedselection function (E, A), means for selecting, at least on said trace,a global recording interval (Ple) overlapping into either side ofdetection interval (Pld), and means for storing the signals of thisrecording interval (Ple) with a determined classification label,according to whether the signals detected therein correspond purely to amicroseismic event or are related, to a certain extent, to artifacts.12) A device as claimed in claim 10, characterized in that the seismicreceivers are hydrophones or geophones or accelerometers distributed inat least one well through the formation. 13) A device as claimed inclaim 11, characterized in that the seismic receivers are coupled withthe formation by a liquid or cement, mechanical coupling means or bymagnetization. 14) A device as claimed in claim 11 or 13, characterizedin that it comprises at least one transmission cable connecting theseismic receivers to recording device (6). 15) A device as claimed inany one of claims 11 to 14, characterized in that it comprises datareception and/or transmission means using a technology based on opticalfibers. 16) A device as claimed in any one of claims 10 to 15,characterized in that it comprises means for inhibiting several recordtraces so as to select only seismic events that are not caused by saidsource or to prevent saturation of recording device (6).